Transmission storage tube



Aug. 29, 1961 J. LEMPERT 2,998,541

TRANSMISSION STORAGE TUBE Filed July 29, 1958 l l F O L ZL F g. I

INVENTOR Joseph Lempert ATTORNEY United States Patent 2,998,541 TRANSMISSION STORAGE TUBE Joseph Lempert, West Elmira, N.Y., assignor to Westlnghonse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed July 29, 1958, Ser. No. 751,657 6 Claims. (Cl. 315-12) This invention relates to cathode-ray tubes, and more particularly to transmission type storage devices. The most common type of transmission type storage device is a direct viewing storage tube. These devices are normally used in display systems such as television and radar in order to obtain brighter images and obtain greater signal to noise ratio. This invention is particularly directed to the storage structure utilized in such tubes.

The conventional type storage tube consists of a foraminated storage structure which consists of a storage contrast control grid of conductive material having a layer of dielectric material disposed uniformly over one surface to form a foraminated structure. A collector 'grid is normally provided on the side of the contrast control grid on which the dielectric coating is provided. A phosphor display screen is provided on the opposite side of the contrast control grid with respect to the collector grid. An electron gun for providing a small pencil-type electron beam is provided on the same side of the control grid as the collector grid for writing a charge pattern on the dielectric coating. A space distributed charge image is produced on the surface of the storage grid by modulating the writing electron gun. The charge image or pattern thus recorded on the control grid controls or modulates the flow of electrons from a flooding type reading gun provided on the same side of the control grid as the writing gun. A visual light image or pattern corresponding to the charge pattern on the storage grid is thus produced on the display screen.

In general the information is written on the storage I grid by the writing electron beam and is performedby either electron bombardment induced conductivity or secondary emission. the dielectric surface with electrons having an energy between first and second crossover potential of the dieelectric material on the storage grid. The storage surface of the grid is thus charged in a positive direction since a greater number of secondary electrons are released than there are primary electrons incident on the surface. These secondary electrons are collected by the collector grid. In this manner the storage of information is accomplished by modifying the potential of the dielectric sur face of the storage grid with respect to the conductive back plate thus altering the charge across the capacitance which is represented by the insulating film of a given thickness.

In general, the information is read off the storage material by using the flooding beam which provides a uni- The usual method is to bombard formflooding electron beam over the entire area of the storage grid of substantially the same electron energy and also normal to the surface of the storage grid. These flood electrons are first accelerated by the electron optical system and are then decelerated so that they approach the storage grid at substantially zero velocity. The electric fields in the vicinity of the storage surface are disposed so that areas of the storage grid which have had a positive charge written on them permit electrons to pass through to the screen without striking the surface of the storage mesh. This is accomplished by disposing the fields so that control of the flood gun can be established when all parts of the storage surface are below flood gun potential, similar to the negative control established by a vacuum tube triode. In those areas where thesurface potential was not modified by the writing gun ice 2 the surface will normally be at cutoff potential and the electrons will be repelled to the collector grid. The areas in which the storage grid is at cutoif will correspond to black areas in the final display image while the high light portions of the image will correspond to the areas of the storage grid which have sufiicient positive charge that all available electrons from the flooding gun to these areas will go through the storage mesh. This may be referred to as saturation potential. Shades of gray in the image will corrmpond to intermediate potentials on the storage surface between the cutoff potential and the saturation potential. As indicated previously the surface of the storage grid is normally biased so that it is negative with respect to the flooding gun. The result is that the flooding gun electrons during the reading operation will not land on the storage surface and thus cannot deteriorate the storage information thereon. This type of storage tube is more fully described in M. Knoll and B. Kazan, Storage Tubes and Their Basic Principles, John Wiley and Sons, Inc., publisher, 1952.

The sensitivity of such a storage tube is related to the amount of charge which must be directed at a area of the grid in order to obtain a potential change of suflicient magnitude to be detected on the display screen. Th change of voltage of the surface of the storage grid per given increment of chargemay be expressed in the following equation.

where AQ is an increment of charge leaving the storage surface, C is the capacitance per unit areaof the. storage electrode and AV is the increment of voltage corresponding to the increment of charge AQ. It is evident from the above equation that a decrease in the capacity per unit area of the storage grid will increase the increment of voltage change corresponding to a given increment of charge. By reducing the capacitance of the storage grid one is able to obtain an increase in sensitivity and a gain in writing speed. The presently known tubes are limited in writing speeds by the storage charge required for unit change in the output.

The writing speed and sensitivity of the storage grid depends on the capacitance of the storage mmh which is inversely related to the thickness of the dielectric layer on the conductive mesh and is directly proportional to the dielectric constant of the dielectric layer. Practical considerations the thickness of the dielectric layer and a decrease in the dielectriccon stant of the dielectric material.

It is therefore an object of this invention to provide an improved transmission storage grid structure. It is another object to provide an improved storage'conf trol structure of high sensitivity. T. It is anotherohject of this invention to provide a direct viewing storage device having an expanded halftone range of operation. a It is another object to provide an improved storage control structure to permit high Writing speed. These and other objects of this invention will be apparent from the following description taken in accordance with the accompanying drawings which form a part of this application, and in which:

FIGURE 1 is a schematic showing of a charge storage tube embodying the teaching of this invention:

FIG. 2 is an enlarged view of a storage structure illus trated in FIG. 1; FIG. 3 is a modified storage structure which maybe incorporated into FIG. 1; and t i 5 FIG. .4 is a modified storage structure which may :be incorporated into FIG. 1. 1 Referring in detail to FIGS. 1 and 2, there is shownxa d c ewin ora t b in po a g nventio The direct viewing storage tube comprises an evacuated envelope 10 of suitable material such as glass and substantially tubular in shape. Positioned at one end of the envelope 10 is a centrally located neck portion 12 for housing an electron gun 14 for providing a flooding type electron beam. Also positioned at the same end of the envelope is a second neck portion 20 positioned off the axis of the envelope 10 for housing a second electron gun. 22 for providing a pencil-type electron beam for writing information. Suitable deflection means illus- 10 trated as electrostatic plates 33 are provided for deflecting the electron beam to scan a raster with the writing be m- Positioned at the opposite end of the envelope 10 is a viewing window 30 which may be also of a light trans- 15 mining-material such as glassv and which provides the support for a light emitting screen. The light emitting screen is a coating 32 on the inner surface of the face plate 30 of a suitable phosphor material such as zinc sulfide activated with silver which emits light in response to electron bombardment. A conductive electrode is provided on one surface of the phosphor layer 32 and may be in the form of a light transmitting coating of conductive material such as tin oxide provided on the inner surface of the glass base plate with the phosphor deposited thereon. In the specific embodiment shown, the electrically conductive electrode is provided by a coating 34 of an electron permeable material such as aluminum deposited on the exposed surface of the phosphor layer 32. A lead 36 is provided from this conductive coating 34 to the exterior with respect to the dielectric grid 42 is an accelerator and collector grid 50. The accelerator grid 50' is of fine mesh material and is provided with a lead-in to the exterior of the tube which is connected to a potential source 52 of the order of 250 volts positive with respect to ground. A coating 54 is also provided on the inner surface of the envelope 10 between the electron guns 14 and 22 and the storage grid structure 40. The coating 54 is of an electrically conductive material and is connected to a potential source 56 so as to apply a potential of the order of 120 volts positive with respect to ground.

The writing gun 22 is of any suitable type for providing a pencil-type beam and consists of at least a cathode 26 and a control grid 28. The cathode 26 of the gun 2 is connected to the movable contact of a single throw switch 31. The fixed contact of the switch 31 is con.- nected to the negative terminal of a battery 29 which may be of a potential of the order of 1500 volts. The positive terminal of the battery 29 is connected to ground. The control grid 28 of the writing gun 22 is connected through a resistor 23 to the negative terminal of a battery 25 which may be of a potential of about 5 volts with the positive terminal connected to ground. A signal source 27 is connected to the control grid 28. The signal source 27 may be of any suitable type which provides a plurality of electrical signals time spaced such as derived from a television set or a radar system and which one desires to display on an image screen.

The flooding gun '14 consists of at least a cathode 16 and a control grid =18. The cathode 16 is connected through a single poled switch 17 to ground.

In order to explain the operation of the device shown in FIG. 1 and for ease of explanation, a table is shown below for the difierent modes of operation with the volt.- ages applied to the respective elements of the storage tube structure.

tron guns 14 and 22 is a storage control structure 40. The storage structure 40 consists of a first grid member 42 which is of a thin insulating material foraminated in structure to provide a plurality of apertures therein and of a suitable material such as a glass film. The grid 42 may be formed in a manner discussed in an article in Material and Methods, page 134, June 1956 by Marshall Byer; Any other suitable dielectric material which may be formed to provide a self-supporting foraminated structure may be utilized. A screen mesh grid 44 of similar area as the dielectric grid member 42 and in general of a much finer mesh is provided adjacent the surface of the dielectric storage grid'42 and between the grid 42 and the electron gun structures 14 and 22. The screen mesh grid 44 is connected by a suitable conductive lead-in to the exterior of the envelope which is in turn connected to the movable contact of a switch 46. The

fixed terminals or contacts of the switch 46 are connected 6 so asto provide two separatesources of potential 45 and 47 which may be connected to the grid 44 depending on the phase of operation. The switch 46 would be connected tothe potential source 45 which is of the order of 15 volts positive withjrespect to ground when the tube 7 Positioned on the-op'positeside of the screen grid During the writing phase the screen mesh 44 is maintained at a positive potential of about 15 volts Withflfir spect to the storage grid 42. The surface of the storage mesh having just been erased may be assumed to be, at zero volts. Thus when the writing beam scans a raster on thestorage grid the electrons will be accelerated through a potential drop of about 500 volts. This potential is between the first and second crossover of the storage grid and therefore more secondary electrons will be emitted from the surface of the storage grid than the number of impinging primaries. This results in a positive charge being deposited on the storage grid 42 in ac, cordance with the intensity and the time of duration of the impinging writing electron beam. The secondary I electrons emitted by the storage grid 42 are collected by the more positive screen mesh 44. It is thus seen that the storage surface of grid 42 will tend to rise toward the potential of the screen mesh grid 44. In high light areas depending on beam current the voltage may. give as much as five volts above zero. The capacitance being charged is no longer represented by the thickness and the dielectric constant of the insulator on the con,- ductive back plate as in prior devices. It is now represented by the dielectric between the storage grid 42 and the screen mesh 44 which is in this case. a vacuum. Thus the dielectric constant of thecapacitor type storage ture 40 is unity. This is the lowestdielectric obtainable and the permit area capacitywhich is rough- 1y inversely proportional to the distance between the ro 1 e in an immediate and practical way by s1m l v ing the distance between the two members. Typical spacings may vary from .0005 inch to .150 inch depending on the requirement of the application. It is of course appreciated that the voltages will vary with the spacing between the members. The accelerator grid 50 is spaced at least ,4 of an inch from the screen mesh 44.

After the information has been written on the dielectrio storage grid 42 the tube may be switched by switch 46 to read operation in which the screen mesh grid 44 is dropped approximately 10 volts in potential which in effect drops the written information on the storage grid 42 below the potential of the flood gun '14 by capacitance coupling. The switch 17 on the cathode 1-6 turns the flood gun 14 on and the switch 31 on the write gun 22 turns the write gun h; The electrons from the flood gun 14 which approach the storage grid 42 are first accelerated by collector mesh 50 and then decelerated to substantially Zero volts as they traverse screen mesh 44 and approach storage mesh 42. These flood gun electrons will pass through those areas of the storage mesh which have potentials of the order of zero Volts or less and will be repelled by those areas having potentials below cut oif, say to 8 volts. Shades of grey will correspond to intermediate potentials.

For the erase portion or phase, the screen mesh grid 44 is brought from 5 to 15 volts, increasing the potential of the storage mesh by the same amount by capacity coupling. The resultant shift in potential of the storage mesh permits electrons from the flood gun 14 to strike the storage surface 42 below first crossover potential and the surface will return uniformly to the approximate potential of the flood gun cathode. The actual potential at which the screen mesh grid 44 should be operated is determined by the tube geometry employed. The operational characteristic of the storage control structure 40 depends upon its geometry and the potential gradients between it and the screen mesh grid 44 and the output screen 32, the operating voltages of the various grids will be related to the spacings employed.

If better control of the potentials of both faces of the storage grid is desired, an additional potential control grid 60 as illustrated in FIG. 3 may be employed. This structure provides in effect three capacitors formed by the following three dielectrics in series: the vacuum dielectric between screen grid 44 and the opposing face of storage grid 42, the dielectric bounded by both sides of storage grid 42, and the dielectric between grid 60 and the adjacent face of storage grid 42. The potential of screen grid 44 will be approximately as indicated in the considerations above. The potential of control grid 60 is adjusted for optimum quality imaging.

An alternative storage control structure is illustrated in FIG. 4 in which a mesh inserted between the storage mesh and the screen is used to control average storage grid potential as the tube changes from one mode of operation. In this figure, a potential control grid 62 is used between the storage grid and the phosphor screen instead of in front of the storage grid as illustrated in FIG. 1. The accelerator grid '50 in FIG. 1 is used as the collector grid and collects the secondary electrons which come oii the storage grid during the writing phase. The typical operating voltages of such a structure are set forth below:

In this embodiment the potential of the surface of the tential by dropping the potential of the potential control screen 62 which drops the potential of the storage grid 42 by capacitive coupling in a similar manner to the operation described with regard to FIG. 1. In this structure because of the capacitance coupling to the collector screen 50 a proportionally greater drop in voltage is required. Thus in FIG. 1 a potential drop of only 5 volts is required while in the embodiment shown in FIG. 3 a potential drop of the order of 50 volts is required. The spacing of the collector screen in FIG. 3 would normally be of the order of 0.125 inch whereas the potential control grid 62 would be of the order of .002 to .025 inch from the storage grid 42.

It should also be noted that one of the principal applications of this storage transmission structure lies in the high sensitivity image devices in which the writing information is coupled from a photocathode onto which a light image is impressed. The sensitivity of these tubes has heretofore been limited by the high capacity of the storage mesh. With low capacity structures herein disclosed, it should be possible to greatly increase the sensitivity. The lower capacity will also increase the writing speed of the standard storage display tubes.

While the present invention has been shown in only a few forms, it will be obvious to those skilled in the art that it is not so limited but is susceptible to various other changes and modifications without departing from the spirit and scope thereof.

I claim:

1. A transmission-type storage tube comprising an envelope and having therein an electron sensitive output screen, electron producing means, an accelerator grid of electrically conductive material positioned between said electron producing means and said output screen, said accelerator grid being of substantially the same area as said output screen and operating at a fixed potential, a storage structure positioned between said accelerator grid and said output screen, said storage structure comprising a thin dielectric film having a plurality of apertures therein, both surfaces of said dielectric film being exposed over their entire area, a potential control grid of electrically conductive material closely spaced to said dielectric film, said dielectric film and said potential control grid forming a storage capacitor with said dielectric film forming one plate of the capacitor and said potential control grid forming the other plate with a vacuum as the dielectric medium, said capacitor having a capacitance of a magnitude that the absolute potential of said dielectric film is established by the potential applied to said potential control grid, and means for directing electrons from said electron producing means onto said dielectric film to establish a charge pattern thereon.

2. A transmission-type storage tube comprising an envelope and having therein an electron sensitive output screen, electron producing means, an accelerator grid of electrically conductive material positioned between said electron producing means and said output screen, said accelerator grid of substantially the same area as said output screen and operating at a fixed potential, a storage structure positioned between said accelerator grid and said output screen, said storage structure comprising a thin dielectric film having a plurality of apertures therein, both surfaces of said dielectric film being eX- posed over their entire area, a potential control grid of electrically conductive material, closely spaced to' said dielectric film and substantially parallel therewith, said electron producing means comprising a first electron'gun means for scanning the surface of said dielectric film with a high energy modulated electron beam to write a charge pattern thereon corresponding to the modulation applied to said first electron gun means and a second electron gun means for flooding said storage structure with low energy electrons, switching means to selectively actuate the first and second electron gun means, potential means connected to said potential control grid to establish a desired reference potential on said storage structure, said output screen substantially parallel to said storage structure and having a conductive portion maintained at a high positive fixed potential so that the electric field produced by said conductive portion of said phosphor display screen penetrates the apertures of said dielectric film to control the transmission of electrons from said second electron gun means through said storage structure, said dielectric film and said potential control grid having a capacitive relationship such that the reference potential of said dielectric film iscontrolled by the potential applied to said potential control grid by said potential means.

3. A transmission-type storage tube comprising an envelope and having therein an electron sensitive output screen, electron producing means, an accelerator grid of electrically conductive material positioned between said electron producing means and said output screen, said accelerator grid of substantially the same area as said output screen and operating at a given potential, a storage structure positioned between said accelerator grid and said output screen, said storage structure comprised of a thin dielectric film having a plurality of apertures therein, both surfaces of said film being exposed over their entire area, and a potential control grid parallel to said dielectric film and spaced a, distance of .0005 inch to .150 inch therefrom so that the potential of said dielectric film is determined by the potential applied tov said potential control grid.

4. A transmission-type storage tube comprising an envelope and having therein an electron sensitive output screen, electron producing means, an accelerator grid of electrically conductive material positioned between said electron producing means and said output screen, said accelerator grid of substantially the same area as. said output screen and operating at a given potential, a storage structure positioned between said accelerator grid and said output screen, said storage structure comprised of a thin dielectric film having a plurality of apertures therein, both surfaces of said film being exposed over their entire area, a potentialcontrol grid spaced from 30005 inch to .150 inch from said dielectric fihn, said electron producing means comprising a first electron gun means for scanning the surface of said dielectric film with a high energy modulated electron beam to write a. charge pattern thereon corresponding to the modulation applied to said first electron gun means andasecond electron gun means for flooding said storage structure with low energy electrons, switching means to selectively actuate the first and second electron gun means, potential means connected to said potential control grid toestablish a desired reference potential on said" storage structure, said output screen maintained at a high positive fixed potential'so that the electric field produced by said output screen penetrates the. apertures of said dielectric film to control the transmission of electrons from said second electron gun means through said storage structure, said dielectric film and said grid having a capacitive relationship such that the reference potential of said dielectric film' is controlled by the potential applied tofsaid grid by said potential means. i 1 a 5'. In a transmission type storage tube of the type ncluding an electron sensitive output screen, a writing electron gun, a flooding type electron gun, an accelerator grid of electrically conductive material positioned between said electron guns and said output screen, a storage structure positioned between said output screen and said accelerator grid, said storage structure comprising a foraminated film of dielectric material and an electrically conductive potential control grid closely spaced to said dielectric film, the method of' operating comprising the steps of: erasing charge stored on said dielectric film by bombarding said film with electrons from said flooding electron gun while said potential control grid is maintained at a potential to control by capacitive coupling the potential of said dielectric film such that the impinging flood electrons land with energies below the first crossover potential of said dielectric film and drive the potential of said dielectric film to that of the cathode of said flood gun, writing a charge image on said dielectric film by bombarding said dielectric film with a signal modulated beam of electrons from said writing gun while said potential control grid is maintained at a potential to control by capacitive coupling the potential of said dielectric film so that the impinging write electrons land thereon with. energies between the first and second cross-over potentials of said dielectric film, and reading said charge image by bombarding said dielectric film with electrons from said flooding gun while said potential control grid is maintained at a potential to control by capacitive coupling the potential of said dielectric film so that said potential is below the potential of said flood gun cathode to allow flood electrons to pass through the apertures in said dielectric film in accordance with the charge thereon written by said writing gun and impinge upon said phosphor display screen.

6. A transmission-type storage display tube comprising a display screen capable of emission of light in response to electron bombardment, a storage structure spaced from said: display screen and parallel thereto, said storage structure comprising a foraminated film of dielectric material, said film having both surfaces exposed, said film having a high lateral resistance, said storage structure further comprising a first potential control grid of electrically conductive material and a second potential control grid of electrically conductive material positioned on opposite sidesof said dielectric film and closely spaced thereto and substantially parallel thereto, an accelerator grid of electrically conductive material positioned on the opposite side of said storage structure with respect to said output screen, a first electron gun means for scanning said storage structure with a high energy modulated electron beam to write a charge image thereon by' electron bombardment between the first and second cross! over potentials of said'dielectric film, said first potential control grid maintained at a potential such that secondary electrons emitted from said dielectric film due to said electron, bombardment are attracted thereto, a second electron gun means for flooding said storage structure with low velocity electrons toread the charge image written on said dielectric film by said first electrongun means,

References Cited in the file of this patent UNITED STATES PATENTS 2,845,561 Sheldon July 29, 1958 

